1. Field of the Invention
The present invention relates to a method for manufacturing a foamed plastics of saturated polyester suitable as a material such as a heat-retaining material or a heat-insulating material.
2. Description of the Related Art
Various methods have been developed as a method for manufacturing a foamed plastics of saturated polyester. In particular, polyethylene terephthalate, polybutylene terephthalate, and the like are excellent in heat resistance and mechanical characteristics, and the development of foams using these materials has been under way.
For example, Jpn. Pat. Appln. KOKAI Publication No. 4-70321 describes a method using an inorganic gas having a boiling point of -50.degree. C. or less as a foaming agent in extruding and foaming a thermoplastic polyester resin. Examples of the inorganic gas are nitrogen, carbon dioxide, helium, neon, krypton, and xenon.
Jpn. Pat. Appln, KOKAI Publication No. 2-49039 describes a method using an inert gas, a saturated aliphatic hydrocarbon, a saturated alicyclic hydrocarbon, an aromatic hydrocarbon, a halogenated hydrocarbon, an ether, a ketone, or the like as a foaming agent in extruding and foaming a polyester resin. Examples of the foaming agent are carbon dioxide, nitrogen, methane, ethane, n-butane, isobutane, n-pentane, isopentane, neopentane, n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane, methylcyclopropane, cyclopentane, 1,1-dimethylcyclopropane, cyclohexane, methylcyclopentane, ethylcyclobutane, 1,1,2-trimethylcyclopropane, benzene, trichloromonofluoromethane, dichlorofluoromethane, monochlorodifluoromethane, trichlorotrifluoroethylene, dichlorotetrafluoroethylene, dimethyl ether, 2-ethoxy-ethanol, acetone, ethyl methyl ketone, and acetyl acetone.
When the above foaming agents are used in extrusion and foaming, the following problems are posed.
(1) An inert or inorganic gas such as carbon dioxide, nitrogen, helium, neon, krypton, or xenon tends to dissipate from the resin. To prevent the gas from dissipating from the resin, the melt viscosity of the saturated polyester resin must be increased. For this purpose, a special additive is used. For example, Jpn. Pat. Appln. KOKAI Publication No. 2-49039 proposes the use of diglycidyl phthalate and a Group I or II metal of the Periodic Table or a compound thereof as such a special additive to improve the melt viscoelasticity of the thermoplastic polyester resin. PA1 (2) When a low-molecular compound of a saturated aliphatic hydrocarbon, a saturated alicyclic hydrocarbon, or an aromatic hydrocarbon, e.g., methane, ethane, propane, n-butane, or isobutane having a boiling point equal to or lower than room temperature at the atmospheric pressure is used, flash ignition and explosion tend to occur in injecting these gases into an extruder because these gases have low boiling points. For this reason, an anti-explosion equipment for the extruder and its periphery is required to result in an increase in installation cost. Problem (1) also occurs in use of these gases. PA1 (3) Compounds having boiling points higher than room temperature at the atmospheric pressure, among saturated aliphatic hydrocarbons, saturated alicyclic hydrocarbons, and aromatic hydrocarbons, such as n-pentane, isopentane, neopentane, n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane, methylcyclopropane, cyclopentane, 1,1-dimethylcyclopropane, cyclohexane, methylcyclopentane, ethylcyclobutane, 1,1,2-trimethylcyclopropane, benzene, dimethyl ether, 2-ethoxy ethanol, acetone, ethyl methyl ketone, and acetyl acetone have an advantage in that the compounds tend not to easily dissipate from a thermoplastic resin during extrusion and foaming. When these foaming agents, however, are used, the expansion ratio can hardly be increased. PA1 (4) Halogenated hydrocarbons, and particularly, chlorine-containing halogenated hydrocarbons such as trichloromonofluoromethane, dichlorofluoromethane, monochlorodifluoromethane, trichlorotrifluoroethylene, and dichlorotetrafluoroethylene are recognized as materials which destroy the ozone layer. Therefore, use of these organic compounds tends to be forbidden in recent years.
As described above, the conventional methods pose problems of flame explosion of a foaming agent, environmental pollution, or necessity of improvements of viscoelastic properties of a saturated polyester resin.